During takeoff and landing, trailing edge high lift devices, located on the trailing edge of airplane wings, are utilized to provide lift and to reduce stalling speed of the aircraft, at the cost of increased drag. Trailing edge high lift devices include surfaces such as flaps, which can move from a stowed position to a deployed position. The flaps may include inboard flaps, located closer to the fuselage, outboard flaps, located further away from the fuselage, and midspan flaps located between inboard and outboard flaps.
Flap control can be provided automatically by a controller within the aircraft or manually by a pilot moving a flaps lever or other control device to a desired position. Manual flap control is traditionally provided by setting a lever to a certain detent, which causes flaps to move to specific positions. For example, a pilot might set a flap lever to a detent such as “flaps 5”, which would cause flaps to move by 25% of their full range of motion. Then, for example, a pilot might set a flap lever to a detent such as “flaps 10”, which would cause flaps to move by an additional 10% of their full range of motion.
Presently, due to weight and spatial constraints, during take-off and landing, most aircraft move all flap surfaces on a wing in unison, with the same increment of their full range of motion for each detent. For example, a single power drive unit provides power equally to inboard and outboard flaps (and midspan flaps if they are present), which causes them to move to the same increment of their full range of motion. While this allows for simpler architecture, and requires only a single power drive unit, it is less than optimal. Due to wing shape, flap location, different airflow at different wing locations and other factors, the optimal amount of incremental motion between detent positions for different flap surfaces is not equivalent. Positioning the flaps to the same incremental motions during takeoff and landing therefore produces sub-optimal drag/lift tradeoffs, which leads to decreased efficiency, increased fuel costs, and increased noise behavior due to flight path.
Presently, there are several methods to compensate for these drawbacks. One method is to determine a “trade-off” or “compromise” position for the flap surfaces, which is a position somewhere between the optimal positions for each flap surface. For example, in an aircraft having inboard, midspan and outboard flaps, if the optimal position for outboard flaps is 10% deflected, while the optimal position for midspan flaps is 13% deflected and for inboard flaps is 15% deflected, a “trade-off” position might be 12% deflection for all flaps. This trade-off provides best drag/lift tradeoffs, given the limitation that the inboard, midspan and outboard flaps are moved to the same increment. However, as the flaps are not in their optimal positions, further advantage could be gained by moving them differentially.
A second method to compensate for this drawback is to have multiple independent power drive units—one for each flap surface or pair of flap surfaces. This produces the benefit that inboard and outboard flaps (and midspan flaps if present) can be optimally positioned, but requires the additional parts and space needed for multiple independent drive trains, which adds weight and complexity to the aircraft.
Other systems exist that have the capability to move various flaps differentially during various phases of flight. However, no such system exists that is designed to move flaps differentially in a manner appropriate for takeoff and landing.
There is therefore a need for methods and systems for providing differential control of flap surface movement utilizing a single drive link to provide improved efficiency over the prior systems during take-off and landing.